KIMBERLITES Kimberley (South Africa). Lecture contents: 1.What are kimberlites – why are they important 2.Where are they emplaced and when 3.Structural.

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Presentation on theme: "KIMBERLITES Kimberley (South Africa). Lecture contents: 1.What are kimberlites – why are they important 2.Where are they emplaced and when 3.Structural."— Presentation transcript:

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1. Kimberlites are a very rare type of magma with extremely deep origin (>150 km). Their key characters are high K, Mg and fluids (CO 2 ) contents. Some kimberlites originate in diamondiferous part of the Earth’s mantle and carry diamonds to the Earth’s surface, which make these rocks economically important. Despite a large interest, kimberlites are still poorly understood rocks due to their ambiguous composition and difficulties in modeling their source and forming conditions following standard geological methodologies. What are kimberlites – why are they important:

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4. What conditions are necessary to yield a kimberlite? 1)Type of ‘solidus’: H 2 O & CO 2 present but in small amounts 2) Phlogopite-bearing peridotite (Potassium enrichment) “METASOMATIC MANTLE” The generation of this type of source-rock is thought to be bond with upwelling of mantle- plumes highly enriched in fluids and incompatible elements (e.g. potassium)

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4. 3) Low degrees of partial melting at very high P, ≥ 40 kbar (below continental shields); This set of conditions could be realized only below a craton or a ‘mobile belt’ So there are 2 types of kimberlites: - On-craton: located in the middle regions of cratonic areas - Off-craton: placed at the boundaries between cratons, on mobile belt terrains (withouth diamond) ;

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4. From these sketches it is evident that the solidus of interest intersects the cratonic geotherm (cold geotherm) where diamond is the stable carbon polymorph, whereas the stable phase would be graphite when below a mobile belt (hot geotherm). [ Kirkley, M., in ‘The Nature of Diamonds’, 1998 ]

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4. Classification of kimberlites: Kimberlites might be divided in three different types based on the geodynamic context of their emplacement. Evidence is found in the type/s of xenoliths occurring in kimberlites. K1: no diamond K2: eclogitic diamond K3: peridotite-type macro- diamonds

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Tectonic setting of kimberlite-bearing areas: The following models have been proposed to describe possible emplacement settings for kimberlite magmas. 1. Lithospheric faults: Lithosphere is crossed by a limited number of deep faults extending towards the upper mantle kimberlites would rise through these fractures, which represents permanent upwelling channels 2. Extension of transform faults: Their localization would be determined by pre-existing continental fractures as leftover of transform faults on continents kimberlites would rise through these fractures; 5.

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Tectonic setting of kimberlite-bearing areas: 5. 3. Hot-spot magmatism: Hot-spot below continental lithosphere with following thinning and rifting kimberlites are emplaced before rifting; 4. Subduction related magmatism: For mature subduction, melting of oceanic material with production of peridotite and eclogite restites. These leftovers would then melt at larger depths to generate kimberlites evidences of kimberlites located parallel to fold belts.

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6. Kimberlite magmas carry xenoliths of peridotite and/or eclogite rocks original of the deepest upper-mantle (maybe even lower-mantle) otherwise not accessible to human investigation. An interesting case study is the reconstruction of the paleo-geotherm of the Slave Craton in Canada. Analyses of eclogite and peridotite xenoliths of the Jericho kimberlite (172 Ma) and high-T-P experiments, aimed to reproduce the phase assemblages of those xenoliths, lead to the reconstruction of the P-T curve below the Slave Craton at the time of Jericho Kimberlite empla- cement (Kopylova et al., 1998). The “stratigraphy” of the paleo-mantle of the craton was also prepared. Kimberlites as natural mantle samplers:

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6. Moreover diamonds often contain inclusion-minerals which are also a source of precious information to reconstruct the composition and the conditions of the mantle below continents at the time of their emplacement. - Mg-wustite (Mg,Fe)O inclusions: Often found in macro-diamonds, they also require a lower-mantle origin - Baddleyte inclusions in diamond (ZrO 2 ): Compositional analyses on these inclusions from diamonds of the Mbuji Mayi kimberlite (Congo) indicate provenience from the lower mantle. Kimberlites as natural mantle samplers:

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7. In different stages of the ore prospection (strategic or tactic) the following methodologies are employed: 1)Airborne and land magnetometry: magnetic anomalies due to the presence of ferromagnetic material (ilmenite) are mapped. Those anomalies could be positive (e.g. South Africa), or negative (e.g. Australia). 2)Airborne and land electromagnets: anomalies due to the presence of conductive material at the surface of kimberlite terrains. 3)Airborne and land gravimetry: crater and diatreme shows negative gravitational anomalies; 4)Radiometric and spectrometric methods: negative and/or positive anomalies due to presence of U, Th and K within respect to the background terrain. Kimberlites ore prospection:

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7. Kimberlites ore prospection: INDEX MINERALS: Garnet: Unusually low in Ca, high Mg, Cr; referred to as G10. Ilmenite: Mg-bearing ilmenite. Diopside: Emerald-green, high-Cr-diopside These minerals are usually found in re-deposited volcanoclastic sediments in rivers, downstream within respect of the actual location of the diamondiferous pipe. A case study is the reconstruction of the “migration” path of microdiamonds and index minerals found in glacial (till) sediments in Northwest Territories, leading to the discovery of the Ekati kimberlite.

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7. Kimberlites ore prospection: New type of prospection: Analyses of natural and induced resorption or/and etching features on natural diamond crystal to deduce fertility of kimberlite pipes. Operated with SEM and in a close future also with AFM (Atomic Force Microscopy). Prof. Yana Fedortchouck (Dalhousie University) is the world leader for these types of investigation. More info from me and at: http://earthsciences.dal.ca/people/fedortchouk/fedortchouk_y.html